Air-cushioned free piston type compressor



Oct. 26, 1965 H. BOLD-r 3,214,085

AIR-'CUSHIONED FREE PISTON TYPE COMPRESSOR combuslian cho h mber rompffssor cyl/nder /7 .Ir/venan' Oct. 26, 1965 H. BoLDT 3,214,085

AIR-GUSHIONED FREE PISTON TYPE COMPRESSOR Filed June 24, 1965 2 sheets-sheet 2 Fig. 5

United States Patent O 3,214,085 AIR-CUSHIONED FREE PISTON TYPE COMPRESSOR Heinz Boldt, Berlin-Charlottenburg, Germany, assignor to Borsig Aktiengesellschaft, Berlin-Tegel, Germany Filed .lune 24, 1963, Ser. No. 290,145

Claims priority, application Germany, June 29, 1962,

B 67,862 4 Claims. (Cl. 230-56) The present invention relates to a free pist-on type compressor the motor part of which consists of parts which are, to a large extent, identical to those used in free piston power gas generators. The term free piston type power gas generator is used to identify a technical apparatus which comprises an air compressor and a combustion chamber which are elements of an open cycle gas turbine process. In contrast thereto, the above mentioned free piston type compressor is a machine which consists of a diesel engine and a piston compressor which compresses any desired gases as, for instance, those employed in the chemical industry.

Free piston machines are known which operate as free piston type power gas generators according to which the heat energy of the hot mixture of combustion g-as and cold scavenging air which was compressed in the same unit with a portion of the heat energy freed during the combustion, is converted into mechanical energy in a gas turbine following said free piston type engine.

There exists the requirement to design free piston type compressors with the same parts employed in connection with free piston type power gas generators while employing also a number of other parts. Furthermore, there exists the requirement to remodel and redesign free piston type power gas generators. As will be evident from the above, a free piston type compressor comprises a motor part and a compressor part while both parts of the unit which are arranged as an image to each other with regard to a transverse central axis, each comprising a power piston, a return piston and a compressor piston, said pistons ,being interconnected by piston rods so as to form a rigid system with each other.

If it is intended to build up .a free piston type compressor with the common parts without taking any further steps, it is necessary to c-ompletely redimension the basic arrangement if the same number of strokes per minute is to be realized.

In view of the larger piston masses, the number of strokes would drop with the work which is stored and remains constant in the chamber located behind the return piston and acting Ias gas spring or cushion. If, however, it is desired t-o retain the same number of strokes which is essential for a favorable relationship between required structural elements and delivered quantity of air, it would be necessary to redesign the engine in such a way that the diameter of the return pistons be increased which, however, would destroy the basic construction of the free piston engine as compressed gas generator. Moreover, it is frequently important to operate the compressor part by means of different gases.

In order to arrive at the building block system so that on one hand the gas generator may be employed for operjation with gas turbines and on the other hand may be employed as driving unit for piston compressors connected thereto, the compressor parts have to be balanced axis, each comprise a power piston, a return piston and a compressor piston which are interconnected by piston rods to form a rigid unit. With this heretofore known system, the suction valves for the compressor parts are arranged on the rear side of the compressor piston while the gas is drawn in during the outward stroke and passes into an overow conduit from which during the inward stroke it is drawn into the working chambers of the compressor parts through specially provided suction valves. The drawback of this system is seen in the fact that the total quantity of gas has to pass through the suction valves twice and moreover when passing through the already hot compressor cylinder is heated up whereby the degree of eiciency of the compressor is considerably reduced. In this connection it may be stated that it is well known with compressors to cool the drawn in gas in order to draw into the cylinder as large a gas weight per stroke as possible. Furthermore, with this system two control slots are required in the compressor part which are located one behind the other when looking in the direction of the piston. One of the control slots controls the start of the compression whereas the other slot determines the start of the work sorage acting upon the rear side of the compressor system, said second slot also serving as overflow slot for the above mentioned drawing in of gas.

It is, therefore, an object of the present invention to provide a free piston type engine which will overcome the above mentioned drawbacks.

It is another object of this invention to provide a free piston type engine which will permit the employment of a free piston type compressed gas generator as driving engine for a piston compressor.

It is still another object of the present invention to provide a free piston type compressor which will be able to maintain the effect of the compressor parts when operating the compressor with different gases.

These and other objects and advantages of the invention will appear more clearly from the following specication in connection with the accompanying drawing, in which:

FIG. 1 diagrammatically illustrates a longitudinal section through a machine according to the present invention, the line X-Y representing the transverse central plane with regard to which the two halves of the unit form an image to each other. For reasons of simplicity only those portions of the right-hand half are shown which due to the unidirectional scavenging ofthe compression chamber do not appear in symmetrical arrangement in the other half.

FIGS. 2, 3 and 4 respectively illustrate by way of diagrams the way of operation and the obtained effects of a free piston type machine according to the invention.

FIG. 5 diagrammatically illustrates a longitudinal section through a machine according to the present invention and, more specically, shows the axial displaceability of the compressor piston and t-he possibilities of varying the control slot.

With a free piston type compressor of the above mentioned type, the desired elfect is made possible according to the invention by creating the inner balance or equalization in the compressor parts as it is necessary for the free piston type principle by means of the back sides of the compressor piston which cooperates with control slots, said back sides of the compressor pistons communicating with the suction conduits of the compressor parts through the intervention of control slots and `gas conduits. Thus, the invention makes it possible to employ an available free piston type compressed gas generator as driving engine for piston compressors. To this end, the back sides of the single acting pistons of the connected compressor parts store that work which gives the compressor parts the character of a machine balanced in itself. In order to be able to maintain the effect of the compressor parts as machines balanced in themselves, also when operating the compressors with different gases, i.e. with gases of different polytrope exponents, it is provided in accordance with the present invention that the location of the control slots is variable in axis parallel direction. Furthermore, in conformity with the present invention, the length of the control slots may be varied. Also, to this end, the compressor pistons may be axially displaceably arranged on -their piston rods.

By varying the length of the control slots and the location of `said control slots, it is possible to increase or decrease the effective piston stroke and thereby to vary the delivered quantity or stored work of the gas spring of the compressor parts. If these or similar structural features are lacking, it is not possible :to adapt the engine to the particular properties of the different gases. If it is desired that the engine ibe universally employable, the above mentioned features are necessary. When the compressor piston is axially displaceable on its piston rod, the clearance or dead space on both sides is changed, thereby varying on one hand the stored work in the chamber behind the compressor piston, which chamber acts as gas spring, and simultaneously in the Working chamber ahead of the compressor piston varying the delivery because with the dead space the course of compression changes and thereby the discharge line becomes longer or shorter.

`Referring now to the drawing in detail, the reference numeral 1 designates the combustion chamber of a prime mover which is designed as free piston engine and prefferably represents a ldiesel engine or a gas diesel engine. The prime mover cylinder Iis designated with the reference numeral 2 and has reciprocably mounted therein a power Ipiston 3. A piston rod 4 connects the power piston 3 with a return piston 5 which acts as scavenging pump piston and is reciprocably mounted in a cylinder 6. The compression chamber 7 for the scavenging air is l-ocated on one side of the piston 5, whereas the chamber 8 for storing the return work for the motor part is located on the other side of piston 5. The chamber 8 acts in the manner of a gas spring, its spring medium being generally formed by air.

25 and 26 are suction and pressure valves for the scavenging and combustion air whic-h is stored in the scavenging air chamber 28. The scavenging and combustion air is admitted to the combustion chamber 1 through the intervention of an inlet opening 27.

Piston rod 9 which extends through a seal 10 connects the return piston 5 to the compressor piston v11 which is reciprocable in a compressor cylinder 12 and which may be adjustable on piston rod 9 in axial direction thereof. A chamber 13 behind the back side 11a of the compressor piston 11 serves for storing the return work for the compressor part. Also this chamber 13 or, more specifically, the medium compressed therein acts as a gas spring. The spring medium in this instance is represented by the gas which is to be compressed in the compressor and which may be air. The working chamber '14 of the compressor is adapted through a valve `17 to communicate with a suction conduit for the gas to be compressed, and is also adapted through a valve 18 to communicate with a pressure conduit 16 for the compressed gas. The engine according to the invention is furthermore provided with a control slot 19 which may be so designed as to be d-isplaceable in axial direction or to be variable as to length. Both possibilities of variation may, if so desired, be provided at the same time. `Branching off from the suction conduit 1S is a gas conduit 20 which through the control slot 19 lead-s into chamber 13 of the gas spring of the compressor part.

IFIG. 5 illustrates how the compressor piston 11 may be axially displaced and also how the control slot 19 may be displaced and varied. Piston 11 is mounted on the piston rod 9 by means of one or more spacer rings 24. In the embodiment of FIG. 5 two such spacer rings are shown. More specifically, the upper half of piston 11 is shown in its outermost left-hand position wher-eas the lower portion of the piston is shown in its outermost right-hand position. lf, for instance, one spacer ring 24 each is mounted at each end of the piston, the middle position of the piston has been realized. Control slot 19 may for instance be designed as a grate with passages 23. The length of the control slots 19 may be varied by means of one or more valve-like closing means 21 which are arranged at each end of the grate. By means of these closing means the corresponding outer passage 23 may be opened or closed as is clearly shown in FIG. 5 (see t-he positions 22 and 22 respectively). The position of the control slot v19 may be varied in axial direction by means of the same closing means 21. This is made possible by alternately opening or closing the same at both ends. Those passages 23 which may be closed do not necessarily have to be arranged on the same portion of the circumference as the remaining control slot. The closing means 21 may be actuated during the operation of t-he free piston type compressors.

The operation and obtained effects will be evident from the diagrams illustrated in FIGS. 2 to 4. However, it is to be understood that these diagrams are given merely by way of example. Referring rst to FIG. 2, A represents the working diagram for the combustion chamber 1 of the prime mover '(diesel engine) with the essential portions al representing the compression stroke, a2 representing the supply of heat by combustion, a3 representing the expansion stroke and a4 representing the scanvenging step.

In FIG. 3, B represents the scavenging diagram for the scavenging air compression chamber 7 with the two essential diagram sections b1=intake and b2=discharge. FIG. 3 furthermore shows the diagram C of the gas spring in chamber 8 which has no delivery but merely shows the pressure going up and down in conformity with the stroke.

FIG. 4 shows the diagram D of the gas spring in chamber 13 which likewise has no delivery but shows the pressure going up and down in conformity ywith the stroke. Also shown in FIG. 4 is the working diagram E of the compressor which for instance compresses from an intake pressure of approximately 20 atmospheres to a compression end pressure of approximately 60 atmospheres. The essential portions of the working diagram E are: e1=in take; e2=compression; e3=discharge of the compressed gas and e4=expansion.

Both sections of the entire unit which are designed as image to each other with regard to the transverse central plane X-Y may be coupled to each other by any standard means in order to obtain a synchronous movement in opposite direction of their two movable systems composed of the rigidly interconnected parts, 3, 4, 5, 9 and 11.

:'In FIG. 5, 19 represents the control slot shaped as a grate consisting of drilled holes 23.

The length of the control slot 19 can be varied by way of one or more valvelike closure means 21 located at each end of the grate and which open the control holes 23 when they are drawn back, as illustrated by position 22 of the valve disc 22.

The locati-on of the control slot 19 can be varied in axial direction `by the same closure means 21 on each side of the control slot 19, namely by opening or closing them alternatively on each side.

The control holes 23 can also be located in other positions on the periphery of the cylinder and can have other congurations as those shown in FIG. 5.

lThe closure means 21 shown as an example in FIG. 5 can be operated from outside during operation of the free piston compressor. The piston 11 is mounted adjustably on the piston rod 9 by means of two spacers 24 as shown Iin FIG. 5. In the drawing the upper half of the piston -11 is shown in its extreme left position, while the lower half of the piston is shown in its extreme right position. By placing one spacer 24 at each end an intermediate position of the piston can be obtained.

'In the example shown in FIG. 5 the piston 11 cannot be adjusted on the piston rod 9 during operation of the compressor.

The pressures prevailing in the individual chambers may, for instance, be as follows:

In chamber `1---approximately l t-o 80 atmospheres;

In chamber 7-approximately 1 to 1.4 atmospheres;

In chamber S-approximately 1 to 1.2 atmospheres;

In chamber -13-approximately 20 to 65 atmospheres and In chamber 14-approximately 20 to 60 atmospheres.

'It is, of course, to be understood that the present invention is, by no means, limited to the particular embodiments shown in 4the drawing but also comprises any modiflcations within the scope of the appended claims.

What I claim is:

1. A free piston type machine structure having two cylinder piston units arranged as an image to each other at opposite sides of a transverse plane of symmetry of said machine structure, in which each of said cylinder pist-on units includes: a prime mover cylinder, a prime mover piston reciprocable in said prime mover cylinder, a return piston cylinder adjacent said prime mover cylinder, a return piston reciprocable in said return piston cylinder, means for entrapping air in said return cylinder on the side of the return piston facing away from said prime mover cylinder to form a gas spring, means for conveying air from said return cylinder on the other side of said return piston to said prime mover cylinder for scavenging purposes, a compressor cylinder on the opposite side of said return piston cylinder from said prime mover cylinder, a compressor piston reciprocable in said compressor cylinder and having a front end face facing away from said return piston cylinder and a rear end face facing said return piston cylinder, piston rod means rigidly interconnecting said prime mover piston and said return piston and said compressor piston, inlet and outlet valve means associated with the end of said compressor cylinder remote from said return piston cylinder, inlet and outlet conduit means respectively leading into and from the said compressor cylinder and respectively controlled by said inlet and outlet valve means, said compressor cylinder having control slot means therein opening into the other end thereof adapted to be controlled by said rear end face whereby the said .other end of said compressor cylinder and the rear end face of the compressor piston form a gas spring, and auxiliary conduit means leading from said inlet conduit means to said control slot means.

I2. A machine structure according to claim 1, which includes means for varying the location of said control slot means in axial direction of said compressor cylinder.

3. A machine structure according to claim .1, which includes means for varying the length of said control slot means.

'4. A machine structure according to claim 1, which includes means mounting said compressor piston on said piston rod for axial adjustability thereon.

References Cited by the Examiner UNITED STATES PATENTS 1/24 Dwyer 230-46 5/51 Welsh et al 230-56 

1. A FREE PISTON TYPE MACHINE STRUCTURE HAVING TWO CYLINDER PISTON UNITS ARRANGED AS AN IMAGE TO EACH OTHER AT OPPOSITE SIDES OF A TRANSVERSE PLANE OF SYMMETRY OF SAID MACHINE STRUCTURE, IN WHICH EACH OF SAID CYLINDER PISTON UNITS INCLUDES: A PRIME MOVER CYLINDER, A PRIME MOVER PISTON RECIPROCABLE IN SAID PRIME MOVER CYLINDER, A RETURN PISTON CYLINDER ADJACENT SAID PRIME MOVER CYLINDER, A RETURN PISTON RECIPROCABLE IN SAID RETURN PISTON CYLINDER, MEANS FOR ENTRAPPING AIR IN SAID RETURN CYLINDER ON THE SIDE OF THE RETURN PISTON FACING AWAY FROM SAID PRIME MOVER CYLINDER TO FORM A GAS SPRING, MEANS FOR CONVEYING AIR FROM SAID RETURN CYLINDR ON THE OTHER SIDE OF SAID RETURN PISTON TO SAID PRIME MOVER CYLINDR FOR SCAVENGING PURPOSES, A COMPRESSOR CYLINDER ON THE OPPOSITE SIDE OF SAID RETURN PISTON CYLINDER FROM SAID PRIME MOVER CYLINDER, A COMPRESSOR PISTON RECIPROCABLE IN SAID COMPRESSOR CYLINDR AND HAVING A FRONT END FACE FACING AWAY FROM 